A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells (SOFC) use a hard, ceramic compound of metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in solid oxide fuel cells, an oxidizing agent, such as 02, is reduced to oxygen ions (02−) at the cathode, and a combustible gas, such as H2 gas, is oxidized with the oxygen ions to form water at the anode.
A SOFC fuel cell comprises a stack of fuel cell units. A SOFC fuel cell unit consists of two major components, a cathode-anode-electrolyte-unit, also referred to as CAE-unit, and an interconnect, having the form of a cassette in some cases. The interconnect serves to connect the CAE-unit of one fuel cell unit electrically to the CAE-unit of another fuel cell unit, so that the electrical power that each CAE-unit generates can be combined. Such interconnects have in planar High-Temperature Fuel Cells (SOFCs) the function to electrically connect the CAE-unit as well as to transport the combustible gas and the oxidizing agent to the respective electrodes of the CAE-unit.
Because the interconnect is exposed to both the oxidizing and reducing side of the CAE-unit at very high temperatures of about 500° C. up to 1100° C., interconnects are one of the critical issues of solid oxide fuel cells. For this reason, ceramics have in the past been more successful in the long term than metals as interconnect materials. However, these ceramic interconnect materials are very expensive as compared to metals. While metal interconnects are relatively easy to fabricate and process, they generally suffer from high power degradation rates partly due to formation of metal oxides, such as Cr2O3, at an interconnect-anode/cathode interface during operation. Nickel- and steel-based alloys are becoming more promising as lower temperature (600-800° C.) SOFCs are developed.
U.S. Pat. No. 7,632,586 B2 discloses an interconnect for a combustible gas and an oxidizing agent. The planar CAE units are positioned one above the other with interconnecting layers formed as planar metal plates arranged in between neighboring CAE units. The respective passages for fuel and oxidant are formed in the anode and cathode layers.
Due to the very high operating temperatures of a SOFC fuel cell stack the effects of thermal expansion and the thermomechanical behavior of the CAE unit and the interconnect structures for supplying the CAE unit with the reactants and conducting the reactants away therefrom have to be taken into account. In particular, the gas distribution structures may undergo some creep, which affects the distribution of flows in the fuel cell. Moreover, the electrodes and interfaces tend to degrade as soon as excessive temperatures are reached.
U.S. Pat. No. 6,670,068 B1 discloses a SOFC fuel cell stack. Thus a plurality of CAE units are in electrically conductive contact with an interconnector, the interconnector comprising a contact plate and a fluid guiding element which is formed as shaped sheet metal part and connected to the contact plate in a fluid-tight manner by welding or soldering. Thereby the contact plate defines a fluid chamber having a combustible gas or an oxidizing agent flowing through it during operation of the fuel cell unit. The shaped sheet metal part is disposed with a plurality of corrugations giving it a wave-like structure. The wave-like structure as such may compensate for some of the thermal expansion of the CAE unit and of the fluid guiding element in operation. However due to the local contact of the wave peaks or wave troughs with the respective electrode, the fluid guiding element has to follow the thermal expansion of the electrode. If the fluid guiding element does not have sufficient elasticity the strain due to thermal expansion is introduced into the electrode. The electrodes are formed from solid, brittle ceramics. Thus, if a high strain is introduced into the electrodes, cracks may be formed, which will ultimately destroy the electrode. In addition the welding or soldering connection provided between the fluid guiding element and the anode also contributes to the stiffness of the construction. In particular if materials having a different coefficient of thermal expansions are used, the strains may finally lead to damages of the electrode and may damage the cell membrane concerned. In particular the flow of reactants may be altered or direct mixing of them can occur if the cell membrane is broken, leading to spontaneous combustion. Thus locally hot spots may form, which may induce local thermal expansion and thus further development of local stress.
Therefore, there is a need for development of improved interconnects for solid oxide fuel cells, addressing one or more of the aforementioned problems, so that more reliable and efficient solid oxide fuel cells are achieved.
Thus it is an object of the invention to improve existing SOFC fuel cells, to make them more reliable, and to allow cheaper manufacturing.